This invention relates to coolable rollers in general and more particularly to an improved design of such a roller.
Coolable rollers with a supporting inner cylinder consisting of metal, on which is firmly disposed a tubular roller jacket, which consists of plastic and has slots as its inner circumference are described in German Petty Pat. No. 76 00 746. In the device disclosed therein, the inner cylinder has the form of a smooth cylinder and coolant ducts in the shape of slots are machined into the inside of the roller jacket facing the inner cylinder. The coolant is brought in through canals in the interior of the inner cylinder which lead to the coolant ducts provided in the roller jacket. The coolant ducts are arranged helically around the inner cylinder.
In the operation of a roller with a roller jacket of plastic, a characteristic temperature distribution adjusts itself. Under the usual operating conditions, the zone of highest temperature is not located on the outside or the inside of the roller jacket, but in the interior of the latter. For a temperature distribution which is brought about by the so-called "working energy" is superimposed on the temperature gradient determined by the temperatures on the outside and inside. In passing through the roll gap, the somewhat elastic plastic jacket is continuously subjected to deformation, as the result of which the plastic is heated up. Due to the deformation conditions, most of the working energy converted into heat per unit volume and time is now produced in the interior of the roll jacket. At the same time, the heat removal conditions for plastic are, of course, the worst in the interior of the roller jacket. This leads to a temperature distribution with a maximum in the interior of the roller jacket, which manifests itself in the fact that destruction due to overheating always occurs first in the interior of the roller jacket and not perhaps at its surface.
In the embodiment described in German Petty Pat. No. 76 00 746, the coolant canals, thus, should have dimensions such that the coolant can be brought very close to or into the zone of maximum temperature in the roller jacket. With a wall thickness of the roller jacket of 30 to 35 mm, the ducts should have a height of 5 to 15 mm, for example. The coolant ducts therefore must extend rather deep into the roller jacket in order to remove the heat from the zone where it is primarily generated. This is true particularly because of the fact that the thermal conductivity of plastics decreases with temperature, which makes the removal from the critical zones more difficult. Since the roller jacket may be subjected on its outside to high line pressures of up to 150 kg per cm of roller length and in isolated cases of up to 200 kg/cm, it is unevenly supported from the inside, i.e., by the lands remaining between the slots, for one, and next to that, practically not at all, as the cooling liquid flowing through the slots is at a low pressure which is just sufficient to move it through the slots. Increasing the pressure of the cooling water to an extent that would counteract the line pressure appreciably, however, is not possible because of the deformation of the elastic plastic roller jacket accompanying this and the leakage resulting therefrom.
The uneven support of the roller jacket from the inside in a zone which has particularly high temperatures and shows a tendency of plastic deformation, leads to markings on the surface of the roller by the canals under the line pressure. Such markings are extremely undesirable, of course, for any kind of processing, which is obvious, for instance, in the case of treating paper webs.